Health system reforms in Colombia and Brazil introduced policies that promote social participation in health care quality control. The objective of this paper was to analyze users' and leaders' perceptions of their capacity to achieve changes in the countries' health services. An exploratory and descriptive qualitative study was performed with individual interviews and focus groups of users and leaders in Colombia and Brazil. Most informants perceived themselves as having the capacity to induce changes to improve the quality of health services. They attributed this capacity primarily to internal factors related to their participatory behavior, plus only one external factor, the availability of institutional participatory spaces. The lack of capacity to influence change was only related to passive attitudes and fear of retaliation in Colombia. The existence of a population with high self-perceived capacity to achieve change appears as a potential factor to be strengthened, while institutional democratic space needs to improve in order to enhance the effectiveness of social participation in health policies.